Conventionally, differently from a primary battery having no charge ability, a rechargeable secondary battery having charge and discharge characteristics is actively under study with the development of advanced technologies including digital cameras, cellular phones, laptop computers, hybrid cars, etc. Examples of the secondary battery include a nickel-cadmium battery, nickel-metal hydride battery, nickel-hydrogen battery, a lithium secondary battery, etc.
Of the above-mentioned secondary batteries, a lithium secondary battery has an actuation voltage of 3.6V or more. The lithium secondary battery may be utilized as a power source for portable electronic appliances, or may be utilized in high-power hybrid cars when a plurality of lithium secondary batteries are connected in series. Since the lithium secondary battery has a higher actuation voltage three times that of the nickel-cadmium battery or nickel-metal hydride battery and also, has superior energy density per unit weight, the use of the lithium secondary battery is rapidly increasing.
At present, a lithium ion battery has been fabricated, wherein a cathode and an anode, which are insulated by a separator interposed therebetween, are wound into a cylindrical or prismatic electrode assembly, and after the resulting electrode assembly is inserted into a metal can, an electrolyte is injected into the metal can. As the metal can is sealed, the fabrication of the lithium ion battery is completed.
More particularly, a conventional lithium ion battery includes a cathode in which a cathode active-material coating layer is provided on one surface or both surfaces of a cathode collector, and an anode in which an anode active-material coating layer is provided on one surface or both surfaces of an anode collector, the cathode and anode being wound with a plurality of separators interposed therebetween.
In the case where active-material coating layers are provided on both surfaces of an electrode collector, the active-material coating layer provided on one surface of the electrode collector is generally shorter than the active-material coating layer provided on the other surface of the electrode collector. Typically, it is desirable that a length and width of an anode be longer than a length and width of a cathode, to prevent extraction of lithium ions from the cathode.
FIG. 1 is a sectional view of a conventional battery, and FIG. 2 illustrates a “jelly-roll” configuration of the wound battery. Considering the configuration of the conventional battery in detail with reference to the drawings, the battery includes a cathode in which cathode active-material coating layers 20a and 20b are provided on at least one surface of a cathode collector 10, an anode in which anode active-material coating layers 40a and 40b are provided on at least one surface of an anode collector 30, and a plurality of separators 50a and 50b interposed between the cathode and the anode.
At least one of a winding beginning portion and winding ending portion of the cathode collector 10 or anode collector 30 contains a cathode uncoated part 10′ or anode uncoated part 30′ where no electrode active-material coating layer is present. These uncoated parts 10′ and 30′ are provided with electrode leads 60 and 70 to be connected to exterior terminals. Both the electrode leads, i.e. a cathode lead 60 and an anode lead 70 are arranged in the same direction.
When the cathode active-material coating layer 20a comes into contact with the anode with the separator interposed therebetween, the cathode active-material coating layer 20a must overlap the facing anode active-material coating layer 40b (in other words, must have a smaller area than that of the anode active-material coating layer 40b), in consideration of a winding deviation and positional change caused upon charge and discharge of the battery. Under this condition, the boundary between the cathode active-material coating layer and the cathode uncoated part 10′ comes across the anode active-material coating layer 40b. This causes micro-holes or shrinkage and damages to other functions of the facing separator 50, resulting in significant heat emission upon contact between the anode active-material coating layer 40b and the cathode uncoated part 10′.
Meanwhile, when the anode and cathode active-material coating layer come into contact with each other under the occurrence of short circuit, there exist a negligible short circuit current and heat emission because of a high electric resistance of the cathode active-material coating layer. However, when the anode comes into contact with the cathode uncoated part (i.e. a part of the cathode collector where no cathode active-material coating layer is present), an insufficient electric resistance causes a serious short circuit current and heat emission which act as dangerous factors significantly deteriorating safety of the battery.
To solve the above-described problems, conventionally, a method for providing insulators 90a, 90b, 90c and 90d at the boundary of the cathode active-material coating layer for preventing short circuit in facing region between the cathode uncoated part and the anode has been adopted.
Referring to FIG. 2 illustrating the jelly-roll wound configuration of the battery shown in FIG. 1, front and rear sides of the cathode lead 60 face the anode uncoated part or the anode active-material coating layer, respectively, with only one layer of separator interposed therebetween. Therefore, either side of the cathode lead 60 has a necessity for an additional insulator for the purpose of preventing short circuit.
Further, since a beginning portion or ending portion of the cathode active-material coating layer faces the anode active-material coating layer with only one layer of separator interposed therebetween, it is necessary to provide an insulator therebetween for the purpose of preventing short circuit.
With respect to the anode lead 70, it comes into contact, at opposite sides thereof, with the anode with six layers of separators and two layers of separators interposed therebetween, respectively. Therefore, there is no necessity for an additional insulator. Similar to the anode lead 70, both surfaces of a winding beginning portion of the anode come into contact with another region of the anode with six layers of separators and two layers of separators interposed therebetween, respectively, eliminating a necessity for an additional insulator.
However, a beginning portion and distal end of the anode coating layer faces the cathode uncoated part or cathode active-material coating layer with only one layer of the separator interposed therebetween and therefore, it is necessary to provide an additional insulator due to a risk of short circuit.